This project is designed to provide information about the motor units that make up large limb muscles in mammals, and of their motoneurons. Recent work has concentrated on the electrophysiological and morphological characteristics of the dendrites of spinal cord motoneurons, particularly on neuroanatomical studies and computer modeling of individual, functionally-identified motoneurons. We have developed a computer simulation approach to identify the fundamental factors that control dendritic morphology. These are used in a relatively simple stochastic (Monte Carlo) simulation that can reproduce a wide range of statistical properties of actual motoneuron dendrites. We have tried to extend the approach to reproduce the quantitative characteristics of dendritic trees in three dimensions, in order to explore whether the 3D anatomy of dendrites results, at least in part, from factors intrinsic to the dendrites themselves. This simulation approach and the data that underlie it are also being used to test ideas about whether dendritic morphologies are optimized for particular functions while minimizing factors that can be regarded as biological costs (see also Project Z01 NS 02079-24 LNLC). A second aspect of this project concerns estimation of the specific membrane resistivity and capacitance of neuronal membranes by reconciling anatomical reconstructions with electrophysiological data from the same motoneurons. In order to facilitate this procedure, we require a simplified electrical representation (called an "equivalent cable") of the complex branching tree of motoneurons that nevertheless embodies the major electrical properties of the fully branched dendritic tree. We have developed an improved algorithm to construct equivalent cables based on the electrotonic length of dendritic segments ("lambda cables"). We have also developed two new cable representations, one based on the attenuation of currents flowing out into the dendritic tree ("attenuation cables") and the other on the time delay inherent in the somatofugal propagation of transient voltage perturbations ("delay cables"). Given the morphology of real neurons, each cable represents a different compromise. We have found that attenuation cables best match the steady-state input conductance and transient response behavior of fully branched neurons. Lambda cables are next best and delay cables give relatively poor matches to both characteristics.